1. Field of the Invention
The present invention relates to a radar system using beamed and reflected radio-frequency energy for detecting objects, measuring distance or altitude, and other purposes, based on detection of the time interval between transmission of the energy (electromagnetic wave) to an object in the beam's path and reception of the energy reflected from the object.
2. Prior Art
A typical conventional radar system comprises an electromagnetic wave transmission means for intermittently emitting electromagnetic waves of pulse waveform, electromagnetic wave reception means for receiving the electromagnetic wave reflected from an object and a distance calculating means for calculating the distance of the object from the radar system on the basis of the time interval between the transmission of the electromagnetic wave and the reception of the same electromagnetic wave reflected from the object. The method of calculating the distance in this kind of radar system is as follows.
As shown in FIG. 11A, an electromagnetic wave received by the electromagnetic reception means is converted into a voltage corresponding to its intensity. A time interval between the emission of the electromagnetic wave and the moment of the voltage reaches a reference voltage V0 is then detected. The detected time interval is multiplied with a half of light speed to calculate the distance of the object from the radar system. However, the electromagnetic wave emitted from the electromagnetic wave transmission means has a gradual pulse waveform having round corners at pulse edges. Therefore, its corresponding voltage curve is a mountain-like waveform as shown by a curve L1 or L2 in FIG. 11A. For this reason, there is a significant amount of time lag (delay) between the reception of the electromagnetic wave by the electromagnetic wave reception means (i.e. time t0) and the moment the voltage reaches a reference voltage V0 (i.e. time t1 or t2). Furthermore, the length of this time lag varies depending on the intensity of the electromagnetic wave received. More specifically, in the example of FIG. 11A, the curve L1 representing a strong electromagnetic wave causes a time lag t1 to reach the reference voltage V0 while the curve L2 representing a weak electromagnetic wave causes a time lag t2 to reach the same voltage. The time difference d1 between times t1 and t2 directly results in a measuring error of the radar system.
An effective way of preventing such a measuring error is believed to amplify the voltage curve L so that its maximum value Vmax exists somewhere in a predetermined higher range from V1 to V2 as shown in FIG. 11B. With this amplification, voltage curves L3 and L4 are newly obtained. The curve L3 causes a time lag t3 to reach the reference voltage V0 while the curve L4 causes a time lag t4 to reach the same voltage. The time difference d2 between times t3 and t4 is very small. Therefore, it becomes possible to improve the measuring accuracy when the distance of an object is calculated on the basis of these times t3 and t4. This is a so-called AGC (Automatic Gain Control) method.
However, when two objects to be detected simultaneously exist in the same detection area of the radar system, an AGC based radar system still has a possibility of encountering difficulty in accurately detecting both of these objects. For example, if one of the two objects is an object having a large reflectance and the other is an object having a small reflectance, there is a large possibility that the latter object will not be detected. This is a case where large and small peaks appear on the voltage curves obtained from the received electromagnetic wave respective voltage curves being largely differentiated in proportion to widely different reflectances of two objects. The AGC operation is carried out in such a manner that the amplified maximum value Vmax of the large peak exists in the predetermined higher range from V1 to V2. If the ratio of large peak height to small peak height is larger than V2 : V0, the small peak will not be able to exceed the reference voltage V0 under such an AGC operation. Namely, the presence of the small peak cannot be detected. This is why there is possibility of failing to detect an object having a small reflectance when another object having a large reflectance exists simultaneously in the same detection area of the radar system.
Similar problem will arise when one of two objects is located directly in front of the radar equipment even if two objects have the same reflectance because, an electromagnetic wave reflected from the object located directly ahead of the radar equipment is stronger than an electromagnetic wave reflected from the other object. Thus, the radar equipment may fail to detect the object offset from the center line the radar beam.